Supercalenders arc employed in the papermaking industry to improve the smoothness or gloss of the surface finish of a paper web. Supercalendering is generally a post-production operation that is not performed on-line on the papermaking machine. Supercalenders are usually employed in off-machine applications, in combination with rewinders, to process paper on reels. The supercalendering process is intended for certain high quality printing papers in which it is necessary to improve the surface properties beyond that produced on the basic papermaking machine.
The supercalender consists of a stack of rolls forming multiple press nips through which the paper sheet is passed. Alternate rolls have "soft" surfaces and are stacked between hard metal rolls. The press nip formed between the rolls deforms the soft rolls creating an extended nip surface.
As the rolls turn, the surface of the soft rolls deform as the paper sheet enters into the nip and again as it passes out of the nip. The important deformation is relative to the surface of the hard roll. Thus as a paper sheet is passed through the nip, the sheet experiences a small relative "sliding" of the soft roll surface on the paper sheet surface. This modifies the surface with little or no effect on the bulk of the paper sheet and improves properties such as gloss and smoothness.
The effect is one sided in that only the surface contacted by the soft roll is modified. To create a two sided effect, the supercalender stack will put two steel rolls together in the middle such that the upper stack will present soft rolls to one side of the paper sheet and the lower stack will present soft rolls to the other side of the paper.
It has been found that changes in moisture and temperature have an additional Supercalendering effect on the paper sheet as the sheet passes through the stack. Since the desired effect is on the surface of the paper, it is the surface temperature and surface moisture of the paper that is important. However, excessive temperatures and moisture can have a destructive effect on the paper sheet and, more importantly, on the soft rolls themselves. It is imperative that any attempt to modify the moisture and temperature of the paper sheet passing through the supercalender be well controlled.
Furthermore, on coated paper sheets there is a further limitation to temperature. Excessive temperature will cause a "picking" of the coating onto the rolls as the paper sheets pass through the nip. This effect must be prevented and greatly limits the allowable temperature and thus the amount of Supercalendering that can be accomplished.
In the production of paper, steam can be used to modify the moisture content and temperature of the paper web. Equipment used to add the steam to the paper web is generally referred to as a steam shower. Steam showers can be used successfully in the Supercalendering process as long as careful attention is paid to the volume of steam delivered. The steam that condenses on the paper web serves to increase both the temperature and moisture content of the web; both of which improve the supercalender effect. For practical reasons, steam showers used in the Supercalendering process must limit their steam flow because of the limited capability of the sheet to condense the steam on its surface. The steam showers must also limit their steam flow to ensure that an excessive temperature is not reached which can lead to "picking" of coated paper in the nip of the supercalender rolls. However, the limits for temperature control and moisture content control are not the same. To improve Supercalendering, it would be preferably to increase the surface moisture of the paper web more while raising the surface temperature less than is conventionally possible by condensing steam.
Steam showers providing steam to condense on the paper is the best method of adding moisture evenly and without "mottling" the surface. Unfortunately in this application, the more moisture that is added the greater the increase in the temperature of the paper web. Before the increased benefits of additional surface moisture are realized, the surface temperature has exceeded the limitations imposed by excessive temperature and "picking" considerations.
To take advantage of the greater process potential provided by steam showers on the supercalender, a method is needed to increase the moisture applied to the paper surface without increasing the paper surface temperature and without allowing excess steam to escape to the atmosphere as the paper web passes through the nip.